Ovulation is the event of rupturing and releasing secondary oocyte ovarian cells. It is the phase of a female's menstrual cycle when an egg (ovule) is released from the ovaries.
After ovulation, during the luteal phase, the egg will be available to be fertilized by sperm. Concomitantly, the uterine lining (endometrium) is thickened to be able to receive a fertilized egg.
If no conception occurs, the uterine lining as well as blood will be shed during menstruation.
In humans, ovulation occurs about midway through the menstrual cycle, after the follicular phase. The few days surrounding ovulation (from approximately days 10 to 18 of a 28 day cycle), constitute the most fertile phase.
The time from the beginning of the last menstrual period (LMP) until ovulation is, on average, 14.6 days, but with substantial variation between women and between cycles in any single woman, with an overall 95% prediction interval of 8.2 to 20.5 days.
The process of ovulation is controlled by the hypothalamus of the brain and through the release of hormones secreted in the anterior lobe of the pituitary gland, luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
In the pre-ovulatory phase of the menstrual cycle, the ovarian follicle will undergo a series of transformations called cumulus expansion, which is stimulated by FSH. After this is done, a hole called the stigma will form in the follicle, and the secondary oocyte will leave the follicle through this hole.
Ovulation is triggered by a spike in the amount of FSH and LH released from the pituitary gland. During the luteal (post-ovulatory) phase, the secondary oocyte will travel through the fallopian tubes toward the uterus. If fertilized by a sperm, the fertilized secondary oocyte or ovum may implant there 6–12 days later.
Estrogen levels peak towards the end of the follicular phase, which causes a surge in levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This lasts from 24 to 36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary via the oviduct.
Through a signal transduction cascade initiated by LH, proteolytic enzymes are secreted by the follicle that degrade the follicular tissue at the site of the blister, forming a hole called the stigma.
The cumulus-oocyte complex (COC) leaves the ruptured follicle and moves out into the peritoneal cavity through the stigma, where it is caught by the fimbriae at the end of the fallopian tube (also called the oviduct).
After entering the oviduct, the ovum-cumulus complex is pushed along by cilia, beginning its journey toward the uterus.
By this time, the oocyte has completed meiosis I, yielding two cells: the larger secondary oocyte that contains all of the cytoplasmic material and a smaller, inactive first polar body.
Meiosis II follows at once but will be arrested in the metaphase and will so remain until fertilization. The spindle apparatus of the second meiotic division appears at the time of ovulation.
If no fertilization occurs, the oocyte will degenerate between 12 and 24 hours after ovulation.
The mucous membrane of the uterus, termed the functionalis, has reached its maximum size, and so have the endometrial glands, although they are still non-secretory.
After ovulation, during the luteal phase, the egg will be available to be fertilized by sperm. Concomitantly, the uterine lining (endometrium) is thickened to be able to receive a fertilized egg.
If no conception occurs, the uterine lining as well as blood will be shed during menstruation.
In humans, ovulation occurs about midway through the menstrual cycle, after the follicular phase. The few days surrounding ovulation (from approximately days 10 to 18 of a 28 day cycle), constitute the most fertile phase.
The time from the beginning of the last menstrual period (LMP) until ovulation is, on average, 14.6 days, but with substantial variation between women and between cycles in any single woman, with an overall 95% prediction interval of 8.2 to 20.5 days.
The process of ovulation is controlled by the hypothalamus of the brain and through the release of hormones secreted in the anterior lobe of the pituitary gland, luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
In the pre-ovulatory phase of the menstrual cycle, the ovarian follicle will undergo a series of transformations called cumulus expansion, which is stimulated by FSH. After this is done, a hole called the stigma will form in the follicle, and the secondary oocyte will leave the follicle through this hole.
Ovulation is triggered by a spike in the amount of FSH and LH released from the pituitary gland. During the luteal (post-ovulatory) phase, the secondary oocyte will travel through the fallopian tubes toward the uterus. If fertilized by a sperm, the fertilized secondary oocyte or ovum may implant there 6–12 days later.
Estrogen levels peak towards the end of the follicular phase, which causes a surge in levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This lasts from 24 to 36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary via the oviduct.
Through a signal transduction cascade initiated by LH, proteolytic enzymes are secreted by the follicle that degrade the follicular tissue at the site of the blister, forming a hole called the stigma.
The cumulus-oocyte complex (COC) leaves the ruptured follicle and moves out into the peritoneal cavity through the stigma, where it is caught by the fimbriae at the end of the fallopian tube (also called the oviduct).
After entering the oviduct, the ovum-cumulus complex is pushed along by cilia, beginning its journey toward the uterus.
By this time, the oocyte has completed meiosis I, yielding two cells: the larger secondary oocyte that contains all of the cytoplasmic material and a smaller, inactive first polar body.
Meiosis II follows at once but will be arrested in the metaphase and will so remain until fertilization. The spindle apparatus of the second meiotic division appears at the time of ovulation.
If no fertilization occurs, the oocyte will degenerate between 12 and 24 hours after ovulation.
The mucous membrane of the uterus, termed the functionalis, has reached its maximum size, and so have the endometrial glands, although they are still non-secretory.
Disorders
Disorders of ovulation are classified as menstrual disorders and include oligoovulation and anovulation:- Oligoovulation is infrequent or irregular ovulation (usually defined as cycles of greater than 36 days or fewer than 8 cycles a year)
- Anovulation is absence of ovulation when it would be normally expected (in a post-menarchal, premenopausal woman). Anovulation usually manifests itself as irregularity of menstrual periods, that is, unpredictable variability of intervals, duration, or bleeding. Anovulation can also cause cessation of periods (secondary amenorrhea) or excessive bleeding (dysfunctional uterine bleeding).
- WHO group I: Hypothalamic–pituitary-gonadal axis failure
- WHO group II: Hypothalamic–pituitary-gonadal axis dysfunction. WHO group II is the most common cause of ovulatory disorders, and the most common causative member is polycystic ovary syndrome (PCOS).
- WHO group III: Ovarian failure
- WHO group IV: Hyperprolactinemia
